The Irish Tricolour shouldn't be used for St Patrick's Day.
The Irish Tricolour flag represents the country of the Republic of Ireland, 26 counties of the island, which in fact doesn't include the 6 counties that St Patrick mainly operated in. In fact, it can be quite offensive to many people living in the place St Patrick mainly operated in, due to a civil conflict.
St Patrick also has his own flag - a red saltire on a white background - which deliberately represents him.
Making St Patrick's day a festival of Irish Nationalism specific to the Republic of Ireland makes zero sense, and using a flag which only represents part of the island, especially since many Americans are also descended from Ulster Scots in Northern Ireland.
I'd like St Patrick's Saltire as it would be consistent with England and Scotland which may also be part of a dissolved UK by then. Kinda like how the frenchy countries use tricolours and the germy countries use horizontal tricolours
I think boob harp is my second preference. It looks cool and would be funny to have a boob on our flag.